Refactor expression checking

[vzarytovskii]

1. Split CEs checking into its own file
2. Split seq checking into its own file
3. Split array/list expressions checking into its own file
4. Put common operations into *Ops.fs file.
5. Refactor CE checking - separate nested function into top-level ones, clean up 2k+ lines main checking function.

It's a bit painful to work with now, so I decided to split it up and re-organize.

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[vzarytovskii]

This is ready once passing. After that I will be refactoring CE checking, so it's easier to argue about its perf.

[vzarytovskii]

I intend to merge it once tests are passing, so I can bring next PR of factoring out a bunch of helper functions out of TcComputationExpression function (it's now like 2k lines). Let me know if it shouldn't be done.